1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus having the zoom lens. More specifically, the present invention relates to a zoom lens useful as a photographic (shooting) optical system of an image pickup apparatus, such as a digital camera, a video camera, a television camera, and a silver-halide film camera.
2. Description of the Related Art
In recent years, an image sensor used in an image pickup apparatus, such as a digital camera or a video camera, has an increased number of pixels.
It is desired by the market that a photographic lens used in an image pickup apparatus including an image sensor having an increased number of pixels is a zoom lens having a high resolution and can effectively correct monochromatic (short wavelength) aberration, such as spherical aberration and coma, and chromatic aberration so that the color of an image does not blur under an illumination by a white light source.
In addition, it is desired that a photographic lens used in an image pickup apparatus including an image sensor having an increased number of pixels is a zoom lens having a wide angle of view.
A conventional negative-lead type zoom lens, which is a zoom lens having a wide angle of view, includes a lens unit having a negative refractive power on the object side. A negative-lead type zoom lens like this enables shooting a closest-distance object at a relatively short distance. Accordingly, the above-described negative-lead type zoom lens can have a long back focal length.
Japanese Patent Application Laid-Open Nos. 2002-287031 and 2001-042217 and U.S. Pat. No. 5,710,669 each discuss a negative-lead type zoom lens for a single-lens reflex camera composed of four lens units having, in order from the object side to the image side, negative, positive, negative, and positive refractive powers, each of which is movable during zooming.
In general, the negative-lead type zoom lens is useful for enlarging the angle of view but the lens configuration is remarkably asymmetric.
In particular, in the negative-lead type zoom lens, a lens unit having a negative refractive power is located at a position closest to the object side of all lens units and thus the arrangement of refractive powers is asymmetric with respect to the aperture stop. Accordingly, a large amount of chromatic aberration of magnification (longitudinal chromatic aberration) may occur at the wide-angle end, and a large amount of axial chromatic aberration and chromatic aberration of magnification (lateral chromatic aberration) may occur at the telephoto end.
Accordingly, it is significant for a negative-lead type zoom lens to be capable of effectively correcting chromatic aberration of both a primary spectrum and a secondary spectrum. More specifically, it is significant for each lens unit of the negative-lead type zoom lens to be capable of effectively correcting chromatic aberration.
In general, in order to correct negative chromatic aberration of magnification, it is useful to use a positive lens made of a low-dispersion material having anomalous partial dispersion, such as fluorite in a lens unit located at a position closer to the back side of the zoom lens than an aperture stop, whose height of incidence (the distance from the optical axis) h of a paraxial chief ray into a lens surface is relatively high.
When the above-described positive lens is used, it is necessary to increase the positive refractive power of the positive lens. However, if the positive refractive power of the positive lens is increased, a large amount of aberrations other than chromatic aberration may occur. Accordingly, in this case, it becomes difficult to effectively correct the aberrations.
In order to effectively reduce chromatic aberration, correct various aberrations other than chromatic aberration, and achieve a high optical performance in the entire zoom range by using an optical material having a low dispersion and anomalous dispersion characteristic, it is significant to appropriately set each lens unit constituting a zoom lens. More specifically, it is particularly significant to appropriately arrange a lens having an anomalous dispersion characteristic in a lens unit.
For example, in a zoom lens including lens units having, in order from the object side to the image side, negative, positive, negative, and positive refractive powers, it is significant to appropriately set an Abbe number and a relative partial dispersion of a material of a lens constituting the second or third lens unit.
If the lens units are not appropriately configured as described above, a large amount of chromatic aberration of magnification may occur at the wide-angle end. In addition, chromatic aberration of magnification may greatly vary in the entire zoom range. Accordingly, it may become difficult to effectively correct chromatic aberration of magnification.